Is it legal for Facebook to actively promote a political agenda?
It's a gray area. It's obviously not a first amendment violation because that only applies to governments. And companies do have some first amendment rights to free association.
On the other hand, Facebook advertises itself as a platform for all people, so promoting their political agenda by removing political content they disagree with is possibly illegal (false advertising - they claim they want to connect people and allow them to share information but don't actually let them do it if it is counter to Facebook's political bias) and possibly a violation of their Terms of Service which is kind of ambiguous as to exactly what content is allowed or not allowed.
For the second part about what's the big deal about them influencing politics?,
The big deal (aside from possibly violating false advertising and their ToS contract) is that it creates conflicts of interest.
There's been clear evidence of Facebook downranking conservative news in trending topics, deboosting conservative videos, and removing / downranking certain political content since around 2016 and after (source). The other answer that claims that Facebook donates more to Republicans is based on donations in earlier years most of which were before 2016 and it states that Zuckerberg, himself, has donated more to Democrats. However, Facebook also removes and downranks some far left content as well which is equally as concerning.
One of the conflicts of interest is that political campaigns buy ads on Facebook at the same that Facebook is donating to them. I've actually personally experienced this when running my campaign for state legislature - Facebook blocked me from running ads on their site and donated the maximum amount to my opponent.